This invention relates to a method of manufacturing visible display devices from electroluminescent phosphors and more particularly to a method of making an electroluminescent light source in the form of a thin, flexible, multi-layered assembly, and to a lamp as produced.
An electroluminescent lamp is basically composed of a layer of electroluminescent phosphor material typically of a metal activated zinc sulfide fixed in place by a polymer binder between two conductive layers, one of which is transparent. When an alternating electric field is applied across the conductors, the phosphors are excited and emit photons with almost all of the radiated energy lying within the visible light spectrum. The emission spectrum and wavelength generated by the phosphors is controlled by the activator element such as copper or manganese.
Electroluminescent phosphors and the polymer binders used are inherently hygroscopic and sensitive to moisture. When exposed to high humidity, the luminescent capability of the phosphor particles is diminished, and performance deteriorates. The sensitivity of the phosphor particles to moisture is so strong that exposure even to conditions of low humidity may adversely affect performance and decrease the light output capacity and useful life of the lamp in which the phosphors are incorporated. To effectively reduce the exposure of the electroluminescent phosphors to moisture, an internal desiccant layer may be incorporated in the lamp.
One method of fabricating an electroluminescent lamp is currently employed starting with a conductive non-transparent substrate of, for example, a sheet of aluminum foil upon which is coated an insulating layer of high dielectric constant material such as barium titanate. Then, an electroluminescent phosphor in a polymer binder is deposited over the dielectric layer and oven dried. A transparent conductive coating formed from, for example, indium oxide and/or indium tin oxide is then deposited over the phosphor layer to form the front electrode. Alternatively, the front electrode may be formed from indium-tin-oxide sputtered Mylar film. A busbar having a conductivity greater than the conductivity of the transparent conductive coating is formed adjacent the periphery of the transparent conductive coating, and a thin nylon preformed film is then applied over the busbar and front electrode. The nylon film or other suitable desiccant film acts as an internal desiccant whose purpose is to collect and hold any small traces of moisture left over from the manufacturing process and to scavenge and retain any moisture which may infiltrate the lamp area. Thereafter, the nylon desiccant film is pulled up to position and attach the front lead, and a second lead is attached to the aluminum foil conductive substrate. Alternatively, the front lead may be positioned before applying the desiccant film, but this may present staining problems since the desiccant film also protects the lamp during processing and handling to prevent staining due, for example, to skin oils from the person handling the lamp. Therefore, the preferable method is to apply the desiccant film and peel it back to position the lead on the busbar.
The entire assembly excluding a portion of the connecting leads is then sandwiched between two moisture barrier films such as a polychlorotrifluoroethylene (PCTFE) film which is commercially available from Allied Chemical Co. under the trade name ACLAR.RTM., or a polyester film, both of which are heat laminated to the assembly and to each other around the perimeter of the lamp. As will be appreciated, the current manufacturing procedure is labor intensive and time consuming, and has inherent quality control problems resulting in a considerable number of unusable lamps. Moreover, in current manufacture lifting the nylon desiccator film in order to position and attach the front lead may break the busbar, and/or permit ingress of moisture and/or contaminants which may result in premature lamp failure in the field.
Moreover, the use of nylon desiccant films adds other manufacturing difficulties. These include the necessary precutting which is usually done off-line, and static charge build-up which inherently occurs in the handling of nylon film. This static charge attracts dust, often causing unacceptable lamps. Other problems involve hand tacking and lead attachment when using nylon film.